DICOM files missing after uploading with Firefox in Orthanc Explorer

Open Mozilla Firefox. Upload the files from a study with several dozens of DICOM instances, with the "Upload" tab of Orthanc Explorer. I used the TOUTATIX sample from OsiriX (that contains 2028 files for about 200MB).

What is the expected output? What do you see instead?

The transfer is tagged as successful, but the statistics of the Orthanc core report only 155 instances: